In general, microphones are classified as follows, according to methods converting mechanical vibration into an electrical signal: carbon microphones using electrical resistance characteristics of carbon particles; crystal microphones using the piezoelectric effect of Rochelle Salt; moving coil microphones generating induced current by vibrating a diaphragm, in which a coil of wire is attached, in a magnetic field; velocity microphones using induced current generated when a metal film installed in a magnetic field receives sound waves and is vibrated; and condenser microphones using capacitance varied according to vibration of a diaphragm caused by sound waves.
Herein, the condenser microphone is universally used as a small microphone, but has a problem in that a DC power supply is necessarily required to apply a voltage to a condenser. Lately, to solve such a problem, an electret condenser microphone using an electret; which has a semi-permanent charge, is used, and has advantages in that a structure of a pre-amplifier is simplified by not needing a bias power supply and also its performance can be improved at a lower cost.
Meanwhile, a transmission section of a mobile terminal radiates a radio frequency signal of a large instantaneous power, which is in the range of a few mW to a few W, through an antenna. The radio frequency signal is induced into a line between a microphone and an external sound pressure signal process circuit and then is applied to a junction field-effect transistor (hereinafter, referred to as “JFET”), which is a field-effect transistor (hereinafter, referred to as “FET”), installed in the inside or outside of the microphone.
At this time, if a power of the radio frequency signal applied to the JFET is greater than a predetermined level, the JFET is nonlinearly operated, so as to generate a noise component relative to a peak envelope together with a harmonic wave. Since the frequency band of the peak envelope overlaps with a sound pressure signal of audio frequency in general, the signal of the noise component is amplified with the sound pressure signal and is inputted to the sound pressure signal process circuit, thereby forming the largest component of noise in the microphone.
Therefore, in order to remove such a noise, a microphone used in a mobile terminal, in the case of a single mode, comprises a notch filter using a LC resonator realized by one chip capacitor in the inside, so that radio frequency signals of a predetermined frequency range are blocked.
Meanwhile, a conventional microphone 1 used in a dual-mode mobile terminal, as shown in FIG. 1, comprises a filter 14 generating resonance at two frequency bands in using two chip capacitors C1 and C2. That is, terminals for mobile communications, which are widely used today, can be classified into Mobile Subscriber Radio Telephones of 900 MHz band and Personal Communication Systems (PCNs) of 1800 MHz band. Therefore, the dual-mode terminal must have a function capable of blocking radio frequency signals of both 900 MHz band and 1800 MHz band.
Referring to FIG. 1, an acoustic module is equivalently represented as a variable capacitor CECM and is connected to the gate G of a FET 12 realized by a JFET. A filter 14 realized by a first and a second capacitor C1 and C2 is connected in parallel between the drain D and the source S of the FET 12. Herein, the first capacitor C1 has a capacitance of about 10 pF and functions to remove 1800 MHz frequency components, and the second capacitor C2 has a capacitance of about 33 pF and functions to remove 900 MHz frequency components.
In the case of using such a microphone in a mobile terminal, the output of the FET 12 is transmitted to a sound pressure signal process circuit 16 after passing the filter 14 designed with parallel connected capacitors C1 and C2, and the output of the sound pressure signal process circuit 16 passes a radio-frequency/intermediate-frequency circuit (RF/IF circuit) 18 and is radiated to the air through an antenna. Herein, the parallel connected capacitors C1 and C2 are designed with a chip capacitor C1 and C2, and each of the capacitors C1 and C2 forms an LC resonance circuit together with respective parasitic inductance L existing on the inside, thereby functioning as a notch filter.
FIG. 2 is a graph showing transfer characteristic of each filter in several cases in which the filter shown in FIG. 1 is realized by one capacitor or two capacitors.
In the graph shown in FIG. 2, the horizontal axis represents frequencies in GHz, the vertical axis represents attenuation levels. A dotted line g1 represents a transfer characteristic in a case of having only the second capacitor C2 of 33 pF and shows a rapid attenuation of a signal at about 900 MHz band, and a solid line g2 represents a transfer characteristic in a case of having only the first capacitor C1 of 10 pF and shows a rapid attenuation of a signal at about 1800 MHz band. Also, a dashed-dot line g3 represents a transfer characteristic in a case of having the first and the second capacitor C1 and C2 connected parallel with each other, and shows a great attenuation of a signal at about 900 MHz band and about 2.2 GHz.
However, such a conventional multi-band low-noise microphone has a problem in that only a little variation of the distance between two capacitors affects the resonance filter's center of 1800 MHz to be moved. Another problem is that it is impossible to effectively remove or block noise in a super-radio frequency mode. That is, in a case of using a new mode such as a new frequency band for IMT-2000 service (for example, 2000 MHz band or 2400 MHz band), since having a narrowband blocking characteristic limited within a predetermined frequency band, a conventional circuit can attenuate only electromagnetic noise within a predetermined frequency band but cannot attenuate radio frequency (RF) noise and electromagnetic noise generated within other frequency bands with the exception of a predetermined frequency band. Such a problem is also generated in a mode below an 1800 MHz frequency band.
Further, in order to improve reliability of a mobile terminal, each element of the terminal is required to have a strong resistance to electrostatic discharge. However, the conventional microphone is problematic in that the conventional microphone is easily affected by electrostatic discharge applied from outside. In other words, the mobile terminal must have no damaged internal circuit element at all, either after it experiences electrostatic discharge in the air with a voltage of 15 kV applied thereto in a state where its microphone is grounded, or after it experiences electrostatic discharge with a voltage of 8 kV applied thereto in a state where it is in direct contact with a node for the electrostatic discharge. However, the conventional microphones cannot satisfy the above-mentioned requirement with respect to the ESD applied from outside.